001/*
002 * Licensed to the Apache Software Foundation (ASF) under one or more
003 * contributor license agreements.  See the NOTICE file distributed with
004 * this work for additional information regarding copyright ownership.
005 * The ASF licenses this file to You under the Apache License, Version 2.0
006 * (the "License"); you may not use this file except in compliance with
007 * the License.  You may obtain a copy of the License at
008 *
009 *      http://www.apache.org/licenses/LICENSE-2.0
010 *
011 * Unless required by applicable law or agreed to in writing, software
012 * distributed under the License is distributed on an "AS IS" BASIS,
013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014 * See the License for the specific language governing permissions and
015 * limitations under the License.
016 */
017package org.apache.commons.collections4.comparators;
018
019import java.io.Serializable;
020import java.util.Comparator;
021
022/**
023 * A {@link Comparator Comparator} that compares {@link Comparable Comparable}
024 * objects.
025 * <p>
026 * This Comparator is useful, for example, for enforcing the natural order in
027 * custom implementations of {@link java.util.SortedSet SortedSet} and
028 * {@link java.util.SortedMap SortedMap}.
029 * <p>
030 * Note: In the 2.0 and 2.1 releases of Commons Collections, this class would
031 * throw a {@link ClassCastException} if either of the arguments to
032 * {@link #compare(Object, Object) compare} were <code>null</code>, not
033 * {@link Comparable Comparable}, or for which
034 * {@link Comparable#compareTo(Object) compareTo} gave inconsistent results.
035 * This is no longer the case. See {@link #compare(Object, Object) compare} for
036 * details.
037 *
038 * @param <E> the type of objects compared by this comparator
039 *
040 * @since 2.0
041 * @see java.util.Collections#reverseOrder()
042 */
043public class ComparableComparator<E extends Comparable<? super E>> implements Comparator<E>, Serializable {
044
045    /** Serialization version. */
046    private static final long serialVersionUID=-291439688585137865L;
047
048    /** The singleton instance. */
049    @SuppressWarnings("rawtypes")
050    public static final ComparableComparator INSTANCE = new ComparableComparator();
051
052    //-----------------------------------------------------------------------
053    /**
054     * Gets the singleton instance of a ComparableComparator.
055     * <p>
056     * Developers are encouraged to use the comparator returned from this method
057     * instead of constructing a new instance to reduce allocation and GC overhead
058     * when multiple comparable comparators may be used in the same VM.
059     *
060     * @param <E>  the element type
061     * @return the singleton ComparableComparator
062     * @since 4.0
063     */
064    @SuppressWarnings("unchecked")
065    public static <E extends Comparable<? super E>> ComparableComparator<E> comparableComparator() {
066        return INSTANCE;
067    }
068
069    //-----------------------------------------------------------------------
070    /**
071     * Constructor whose use should be avoided.
072     * <p>
073     * Please use the {@link #comparableComparator()} method whenever possible.
074     */
075    public ComparableComparator() {
076        super();
077    }
078
079    //-----------------------------------------------------------------------
080    /**
081     * Compare the two {@link Comparable Comparable} arguments.
082     * This method is equivalent to:
083     * <pre>((Comparable)obj1).compareTo(obj2)</pre>
084     *
085     * @param obj1  the first object to compare
086     * @param obj2  the second object to compare
087     * @return negative if obj1 is less, positive if greater, zero if equal
088     * @throws NullPointerException if <i>obj1</i> is <code>null</code>,
089     *         or when <code>((Comparable)obj1).compareTo(obj2)</code> does
090     * @throws ClassCastException if <i>obj1</i> is not a <code>Comparable</code>,
091     *         or when <code>((Comparable)obj1).compareTo(obj2)</code> does
092     */
093    @Override
094    public int compare(final E obj1, final E obj2) {
095        return obj1.compareTo(obj2);
096    }
097
098    //-----------------------------------------------------------------------
099    /**
100     * Implement a hash code for this comparator that is consistent with
101     * {@link #equals(Object) equals}.
102     *
103     * @return a hash code for this comparator.
104     * @since 3.0
105     */
106    @Override
107    public int hashCode() {
108        return "ComparableComparator".hashCode();
109    }
110
111    /**
112     * Returns {@code true} iff <i>that</i> Object is is a {@link Comparator Comparator}
113     * whose ordering is known to be equivalent to mine.
114     * <p>
115     * This implementation returns {@code true} iff
116     * <code><i>object</i>.{@link Object#getClass() getClass()}</code> equals
117     * <code>this.getClass()</code>. Subclasses may want to override this behavior to remain
118     * consistent with the {@link Comparator#equals(Object)} contract.
119     *
120     * @param object  the object to compare with
121     * @return {@code true} if equal
122     * @since 3.0
123     */
124    @Override
125    public boolean equals(final Object object) {
126        return this == object ||
127               null != object && object.getClass().equals(this.getClass());
128    }
129
130}